In numerous applications, there is need to amplify a small input signal that is close to zero volts (0V) (or a negative supply voltage) and to add a fixed constant voltage to the result. FIG. 1 illustrates an exemplary battery monitoring circuit 100 in which a small voltage close to a zero volt reference is amplified and offset by a constant voltage. Battery monitoring circuit 100 comprises battery 105, sense resistor 110, scaler 115, analog-to-digital converter (ADC) 120 and load circuit 125. In an exemplary embodiment, load circuit 125 may be a cell phone that monitors battery 105 to determine if battery 105 is charging or discharging and to determine the rate at which battery 105 is charging or discharging.
The battery current generated by battery 105 is sensed over sense resistor 110, which is a very small resistor (about 0.1 ohm). During charging of battery 105, the input voltage to scaler 115 is positive. During discharge of battery 105, the input voltage to scaler 115 is negative. In both situations, the sense voltage is very small (typical range is −100 mV to +100 mV, but −10 mV to +10 mV also is a possible range). This small sense voltage has to be scaled and amplified to a signal that can be used in by ADC 120. Scaler 115 typically amplifies the sense voltage by a factor of 10 or 100 and add the mid-level voltage of ADC 120 to the amplified signal. The digitized sense voltage at the output of ADC 120 is then read by processing circuits in the cell phone (i.e., load circuit 125) to determine the state of battery 105. It should be noted that there are many other applications in which scaler 115 performs a similar amplification and offset function.
Scaler 115 must have a high ohmic input so that a low-pass filter (e.g., a 1M resistor and a 1 μF capacitor) may be added between the sense voltage and scaler 115. In that way, the average battery current can be measured. One approach implementing scaler 115 is to use a standard non-inverting operation amplifier (op amp) having a first resistor, R1, with a first terminal coupled to the negative input of the op amp and a feedback resistor, R2, coupled between the negative input and the output. Two level shifters are used to add a constant voltage (typical half the supply voltage: Vdd/2)) to the positive input of the op amp and to the second terminal of the first resistor, R1. In this approach, the input signal of the op amp is always positive. The transfer function from input to output voltage of such a circuit is:Vout=Vdd/2+((R1+R2)/R1)Vin
Unfortunately, this approach requires very accurate level shifters. Mismatch and non-linearity in the level shifters directly affects the accuracy of the overall scaling. The level shifters must be able to be used with a high impedance input of the scaler, which reduces the number of possible circuits.
There is therefore a need in the art for improved circuits that are capable of monitoring small voltages close to zero volts (or a negative supply rail). There is a further need for improved amplification circuits that amplify a small input signal close to zero volts (0V) (or a negative supply voltage) and add a fixed offset voltage to the result.